1. Field of the Invention
The present invention relates to a loudspeaker, and more particularly to a microspeaker having a good sound quality.
2. Description of the Related Art
Small loudspeakers (so-called “microspeakers”) have been used for playing ringtones on mobile phones, for example. Typically, a microspeaker has a structure as illustrated in FIG. 5. Specifically, a microspeaker 50 includes a diaphragm 51, a voice coil 52 for driving the diaphragm 51, and a magnetic circuit 53 for producing a magnet field that is perpendicular to the voice coil 52. The periphery of the diaphragm 51 is supported by a wall portion of a frame 54. One end of the voice coil 52 is coupled to the diaphragm 51, and the other end thereof is located in the magnetic gap of the magnetic circuit 53. Note that although not shown, both ends of a coil section of the voice coil 52 are extracted to the side of the frame 54 and are connected to a sound input signal section through which a sound signal is input from the outside. The magnetic circuit 53 includes a yoke 56, and a magnet 57 provided on the yoke 56, and an upper plate 58 provided on the magnet 57, and produces a magnet field that is perpendicular to the voice coil 52.
In a microspeaker as described above, a resin film is used as the material of the diaphragm, and the distance between the diaphragm and the upper plate (pole piece) forming a part of the magnetic circuit is very small. As a result, distortion is exaggerated in a particular frequency range, whereby a satisfactory sound quality cannot be obtained.
In order to solve this problem, other microspeakers have been proposed, as illustrated in FIG. 6 and FIG. 7. In the loudspeaker of FIG. 6, a sound absorbing material 69 is provided between a diaphragm 61 and an upper plate 68. In the loudspeaker of FIG. 7, an upper plate 78 is shaped so as to conform to the shape of the diaphragm. These loudspeakers aim to suppress the reflection of sound waves between the diaphragm and the upper plate so as to improve the frequency characteristic and to reduce the harmonic distortion.
However, in either one of the loudspeakers of FIG. 6 and FIG. 7, there is little improvement on the audible sound quality, although the peak/dip in the frequency characteristic is reduced.
In view of the state of the art as described above, there is a strong demand for a small loudspeaker having a good sound quality.